My present invention relates to an apparatus for controlling web tension and, more particularly, to an apparatus which is used in the production of bags with perforation welding tools to control the tension between an upstream pair of feed rollers and a downstream pair of withdrawal rollers.
In the production of bags, for example, a synthetic resin foil web is moved past a perforating-welding head between an upstream pair of feed rollers and a downstream pair of withdrawal rollers. The tension in the web at perforation welding to form the individual bags, e.g. in the production of rolls of such bags, is of importance. To control the tension, a guide roller was provided on one side of the web and on the other side a tension sensor was disposed to determine the actual tension value. This sensor was in the form of a blowing tube which was connected to a compressed air source and directed onto the web through at least one air outlet opening cooperating with a measured value unit which controlled the speed of the feed roller pair or the withdrawal roller pair, preferably the latter.
As noted, in the production of bags with perforated-weld seams it is necessary, especially in the region of the perforations and perforation-welding head that the web tension not exceed a predetermined value. If the web tension is too great in this region, the web segments adjoining one another at the perforation-weld seam tend to pull apart and the weld seam quality is damaged. This can occur at even relatively small web tensions since the foil web in the region of the perforation-weld seam is generally brought to an elevated temperature and is more or less plastified thereby.
To avoid these drawbacks or to keep the danger to the web at the perforation-weld seam as small as possible, it has been found that the bag should be practically in a tension-free condition at the point of the perforation-weld seam formation and/or that the still warm weld seam not be mechanically stressed. This ensures optimal seam quality.
For this purpose, a special sag or looseness control is applied whereby the web passes beneath guide rollers and between them and can be looped through substantially 180xc2x0 and subjected to a compressed air flow. The web assumes a greater or lesser distance from the blowing tube depending upon the pressure and this distance can be measured by a sensor which can output control pulses to the drive of either the feed roller pair or the withdrawal roller pair or both. In this way excessive tensions in the web can cause the withdrawal roller pair to be driven at lower speed to reduce the tension and thus equalize the tension in the web.
It has been found that with the above-mentioned earlier web tension control unit which requires a relatively large looping angle of the web, that significant friction arises with the consequence that slip occurs at the withdrawal roller pair and/or that the sag or looseness does not form at the blowing tube but rather at the feed roller pair. The result is damage to the web, tearing and above all, variations in the bag length and the like. In the travel of the web, significant whistling noise is generated and even the control response is unclean, especially with thick and multilayer foil webs. Because of the offset of the air outlet openings, wavy air cushions are generated which can result in imprecise measurements. Furthermore, with looping of the blowing tube, a second perforation-welding tool or head may be required. In addition, the sag, because of the location of the blowing tube, cannot be readily observed from below. Finally, a matching of the conventional blowing tube to different web widths can be achieved with difficulty and, for example, has required attaching covers by adhesives to the blowing tube at the cost of significant noise development.
It is, therefore, the principal object of the present invention to provide a web tension controller for producing perforation-welded bags whereby the above mentioned drawbacks are largely eliminated and improved bag making results can be achieved.
Another object of the invention is to provide a web tension controller for the aforedescribed purposes which can operate at low cost and with reduced noise development and especially with the production of less whistling-type noise.
These objects are attained, in accordance with the invention, by providing on one side of the web the blowing tube and on the opposite side of the web a device for measuring the position of the web, the air from the blowing tube being emitted from an air outlet opening formed in a wall surface which is substantially parallel to the synthetic resin web. In this manner, the especially disadvantageous looping of the blowing tube is completely avoided.
The wall surface can, for example, be part of a polygonal cross section duct extending beneath the web across its entire width. Preferably the duct is a rectangular cross section duct. The outer side of the wall provided with the air outlet opening forms the surface described previously and may lie in a plane which is tangent to the nadir of a guide roller. When guide rollers are provided upstream and downstream of the air box formed by the duct, the nadirs of these guide rollers define a plane and the surface can lie in that plane. The outer side of the wall surface of the rectangular section tube can be offset slightly in the direction of the guide rollers.
Especially effective results are obtained when the wall surface of the duct has a width in the web travel direction which is somewhat smaller than the spacing between the peripheries of the two guide rollers and when the widths of the air outlet openings, which can extend substantially over the entire length of the duct and the width of the web is about one-fifth of the width of the wall surface of the duct provided with the air outlet openings.
With the invention the web tension control is effected with approximately no evolution of noise at the synthetic resin web. The air cushion between the wall surface of the duct and the web is completely smooth and clean so that optimum tension regulation is assured and the spacing and length of the bags and between the neighboring bags or between the web segments remains constant and correct. The rectangular section duct is especially economical and because of the use of the rectangular section duct, the spacing between its wall surface and the web can be measured or established with a high degree of precision from above with an appropriate measuring instrument, for example an ultrasonic sensor.
Because the air outlet opening in the wall of the duct is an elongated hole or longitudinally extending slit, it is simple to reset the effective length of the opening to suit the width of the web since any unnecessary portion of the opening can be closed by a simple magnetic strip. The web tension controller for producing perforation-welded bags from a web of a synthetic resin foil can comprise an upstream pair of feed rollers and a downstream pair of withdrawal rollers which advance the web past a perforation-welding head at a tension determined by relative speeds of the pairs of rollers. The web tension control can comprise:
a support located between the pairs of rollers;
a guide roller on the support engaging one side of the web;
a device on the support for emitting a flow of air in a direction of the web from an opposite side thereof, the device comprising a wall having a surface juxtaposed with and substantially parallel to the web, an air outlet opening in the wall, and blower tube having an outlet communicating with the air opening for producing the flow of air and thereby deflecting the web; and
a position detector on the support for measuring a position of the one side of the web, thereby determining a value of web tension, and coupled to at least one of the pairs of rollers for operating same to control the web tension.